The Klenow fragment is a truncated form of the DNA polymerase I enzyme from E. coli, which is missing the 5' to 3' exonuclease activity. It was initially discovered by Werner Klenow in 1968 and has since been used extensively in molecular biology research. The Klenow fragment has a 3'-5' exonuclease activity, which allows it to fill in single-stranded gaps in double-stranded DNA molecules. It can also be used to add specific nucleotides to the 3' end of a DNA strand. In addition, it can be used to create blunt ends on double-stranded DNA molecules, which can then be ligated together with other molecules. The Klenow fragment is also useful for creating large amounts of single-stranded DNA for use in sequencing reactions or other applications.